Carbon-based filters are employed in individual and collective-protection applications to provide safe breathing to personnel in a chemically contaminated environment. General purpose filters, such as those employed by first responders and the military, for example, are designed to remove a wide range of toxic vapors. Examples of toxic vapors removed by general purpose filters include, for example, hydrogen cyanide, cyanogen chloride, phosgene, sulfur dioxide, hydrogen chloride and chlorine. The filter, in its simplest form, consists of a housing into which filtration media is located. The contaminated air stream is passed through the bed of filtration media. It is upon contacting the contaminated air stream with the filtration media that toxic vapors are removed by either physical adsorption or by chemical reaction.
Typically, general purpose respirator filters employ activated carbon impregnated with basic metal salts which include, for example, copper-chromium, copper-molybdenum (U.S. Pat. No. 4,801,311), and copper/zinc/molybdenum (U.S. Pat. No. 5,492,882). Examples of basic metal salts include carbonates, hydroxides and oxides. In several instances, triethylenediamine is added to the formulation to promote the removal of cyanogen chloride (U.S. Pat. No. 4,802,898).
Carbon-based filtration media rely on the metal impregnants to facilitate chemical reactions. This is because the contribution of the carbon substrate to the chemical reactions is small. For example, mixtures of copper carbonate/copper hydroxide may be impregnated into the pores of activated carbon to promote stoichiometric reactions involving the removal of acid gases, such as for example hydrochloric acid (HCl):CuCO3+2HCl→CuCl2+H2O+CO2 Cu(OH)2+2HCl→CuCl2+2H2O
From the above reactions, the basic copper salts react with hydrochloric acid to produce the corresponding copper (II) chloride while liberating H2O and, in the case of the carbonate, carbon dioxide (CO2). Note that the above reactions are stoichiometric and not catalytic. That is to say, one copper carbonate or copper hydroxide molecule will remove two molecules of hydrochloric acid. As a result of the stoichiometric nature of the reaction, it is desired to maximize the number of reactive sites associated with the filtration media.
As is well known to one skilled in the art, a well dispersed metal phase is desired in order to maximize the number of sites available to facilitate the reactions necessary for the removal of toxic vapors. Often times, the loading of basic metal salts is limited to approximately 10% by weight metal to 15% by weight metal. Higher metal loadings can result in the formation of large metal particles, leading to pore blockage and a decreased overall effectiveness of the filtration media.
Procedures for impregnating activated carbon with basic impregnants are well known to one skilled in the art. These procedures involve mixing carbonates, hydroxides and/or ammonium complexes of base metals such as for example copper (copper carbonate-hydroxide), molybdenum (ammonium molybdate), zinc (zinc carbonate-hydroxide) and vanadium (ammonium vanadate), in concentrated ammonium hydroxide solutions. Optionally, ammonium carbonate is added. The solution is used to impregnate activated carbon granules to the desired metal loading. Materials prepared according to this procedure do not; however, disperse the metal phase throughout the pore structure. For example, it has been reported that activated carbon impregnated with copper and zinc, or copper, zinc and molybdenum, results in locating zinc primarily at the external surface of the granule, versus a zinc phase well dispersed throughout the pore structure (Rossin and Morrison, Carbon 29, (1991) 887; Rossin and Morrison, Carbon 31, (1993) 657). The net effect is a reduced dispersion of zinc and a poor utilization of the zinc phase.
One mechanism for increasing the effectiveness of a filtration media to facilitate stoichiometric reactions is to employ a functionalized porous media. In this manner, the support, along with the impregnants, will be capable of contributing to stoichiometric reactions leading to the removal of toxic vapors.
Substrates other than activated carbon are known to be useful in removing toxic chemicals. One example is zirconium hydroxide, (Peterson et al., Ind. Eng. Chem. Res. 48 (2009) 1694), wherein zirconium hydroxide was found to remove sulfur dioxide from streams of air in an order of magnitude greater than the value achieved for activated carbon.
The removal of cyanide compounds, such as for example hydrogen cyanide, is of special interest to this invention. Base metal carbonates, hydroxides and oxides of copper and zinc are known by one skilled in the art to be effective in the removal of hydrogen cyanide. For example, hydrogen cyanide is removed upon contact with, for example, copper carbonate according to the following reactions:CuCO3+2HCN→Cu(CN)2+H2O+CO2 2Cu(CN)2→CuCN+C2N2 
According to the first reaction, copper carbonate reacts with hydrogen cyanide to form copper (II) cyanide. Copper cyanide subsequently decomposes to yield copper (I) cyanide, liberating cyanogen, which is highly toxic and therefore an undesired by-product. The addition of molybdenum to the formulation has been shown to minimize the formation of cyanogen; however, cyanogen formation is not eliminated all together. The role of molybdenum in minimizing cyanogen formation is not clear. One possible explanation is that molybdenum complexes with copper, increasing the stability of the copper (II) cyanide complex.
Zinc is much more preferred in applications involving the removal of cyanide compounds, such as for example hydrogen cyanide. Hydrogen cyanide is removed upon contact with zinc carbonate, for example, according to the following reaction:ZnCO3+2HCN→Zn(CN)2+H2O+CO2 
There is no reduction of zinc (II) cyanide to zinc (I) cyanide, as per reactions involving copper as described above. This is because zinc does not possess a stable+1 oxidation state, and, as a result, the product zinc (II) cyanide is stable.
There are issues associated with the use of zinc only in carbon-based filtration media. In order for an impregnant to be effective, the impregnant must be well dispersed throughout the pore structure of the media. As stated above, impregnation of carbon-based media with zinc results in the zinc being confined to a region at or very near the external surface of the granule, leading to a poor metal dispersion and consequently a poor utilization of zinc.
The removal of cyanogen chloride is also of special interest to the present invention. Triethylenediamine (TEDA) is known by one skilled in the art to be effective in the removal of cyanogen chloride. Although not wishing to be bound by theory, it is believed that TEDA catalyzes the hydrolysis of cyanogen chloride as described below:

Reaction product hydrochloric acid, HCl, is then removed via reaction with base metal carbonates, hydroxides and/or oxides, or via reaction with TEDA. Reactions involving HCl and TEDA will poison TEDA. As known by one skilled in the art, the addition of base metal oxides, hydroxides and/or oxides to TEDA impregnated activated carbon will increase the service time of the filter.
Thus an objective of the present invention is to provide an improved filtration media for removing hazardous materials from air streams. Another objective is to incorporate at least one reactive moiety within the filtration media, to enhance the filtration performance. A further objective is to provide a method of using the improved filtration media to remove hazardous materials from air streams.